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                <h1 id="firstHeading" class="firstHeading">Qualified name lookup</h1>
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<p>A <i>qualified</i> name is a name that appears on the right hand side of the scope resolution operator <code><b>::</b></code> (see also <a href="name.html#Qualified_identifiers" title="cpp/language/identifiers">qualified identifiers</a>).
A qualified name may refer to a 
</p>
<ul>
<li> class member (including static and non-static functions, types, templates, etc)
</li>
<li> namespace member (including another namespace)
</li>
<li> enumerator
</li>
</ul>
<p>If there is nothing on the left hand side of the <code><b>::</b></code>, the lookup considers only declarations made in the global namespace scope (or introduced into the global namespace by a <a href="namespace.html" title="cpp/language/namespace">using declaration</a>). This makes it possible to refer to such names even if they were hidden by a local declaration:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
  <span class="kw1">struct</span> std<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
  <a href="../io/cout.html"><span class="kw1453">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"fail<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// Error: unqualified lookup for 'std' finds the struct</span>
  <span class="sy4">::</span><a href="../io/cout.html"><span class="kw1453">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"ok<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// OK: ::std finds the namespace std</span>
<span class="br0">}</span></pre></div></div>
<p>Before name lookup can be performed for the name on the right hand side of <code><b>::</b></code>, lookup must be completed for the name on its left hand side (unless a <a href="decltype.html" title="cpp/language/decltype">decltype</a> expression is used, or there is nothing on the left). This lookup, which may be qualified or unqualified, depending on whether there's another <code><b>::</b></code> to the left of that name, considers only namespaces, class types, enumerations, and templates whose specializations are types:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
  <span class="kw4">static</span> <span class="kw4">int</span> n<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">int</span> A<span class="sy4">;</span>
  A<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>    <span class="co1">// OK: unqualified lookup of A to the left of :: ignores the variable</span>
  A b<span class="sy4">;</span>          <span class="co1">// error: unqualified lookup of A finds the variable A</span>
<span class="br0">}</span></pre></div></div>
<p>When a qualified name is used as a <a href="declarations.html" title="cpp/language/declarations">declarator</a>, then lookup of all names used in the same declarator that follow that qualified name, but not the names that precede it, is performed as if qualified the same way:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> X <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> number <span class="sy1">=</span> <span class="nu0">100</span><span class="sy4">;</span>
<span class="kw1">class</span> C <span class="br0">{</span>
  <span class="kw1">class</span> X <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
  <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> number <span class="sy1">=</span> <span class="nu0">50</span><span class="sy4">;</span>
  <span class="kw4">static</span> X arr<span class="br0">[</span>number<span class="br0">]</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
X C<span class="sy4">::</span><span class="me2">arr</span><span class="br0">[</span>number<span class="br0">]</span>, brr<span class="br0">[</span>number<span class="br0">]</span><span class="sy4">;</span>   <span class="co1">// Error</span>
  <span class="co1">// Every name in the declarator "C::arr[number]" after "C::arr"</span>
  <span class="co1">// is looked up within C::, but the names before C::arr are unaffected,</span>
  <span class="co1">// The names in the second declarator ("brr[number]") are also unaffected</span>
  <span class="co1">// equivalent to:</span>
  <span class="co1">// "::X C::arr[C::number], brr[::number]"</span>
C<span class="sy4">::</span><span class="me2">X</span> C<span class="sy4">::</span><span class="me2">arr</span><span class="br0">[</span>number<span class="br0">]</span>, brr<span class="br0">[</span>number<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// Compiles, size of arr is 50, size of brr is 100</span></pre></div></div>
<p>If the name on the right hand side of <code><b>::</b></code> is a destructor or pseudo-destructor (that is, the character <code><b>~</b></code> followed by an identifier), that identifier is looked up in the same scope as the name on the left hand side of <code><b>::</b></code>
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> C <span class="br0">{</span> <span class="kw1">typedef</span> <span class="kw4">int</span> I<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">typedef</span> <span class="kw4">int</span> I1, I2<span class="sy4">;</span>
<span class="kw4">extern</span> <span class="kw4">int</span> <span class="sy2">*</span>p, <span class="sy2">*</span>q<span class="sy4">;</span>
<span class="kw1">struct</span> A <span class="br0">{</span> ~A<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">typedef</span> A AB<span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  p<span class="sy2">-</span><span class="sy1">&gt;</span>C<span class="sy4">::</span><span class="me2">I</span><span class="sy4">::</span>~I<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// the name I after ~ is looked up in the same scope as I before ::</span>
                 <span class="co1">// (that is, within the scope of C, so it finds C::I)</span>
  q<span class="sy2">-</span><span class="sy1">&gt;</span>I1<span class="sy4">::</span>~I2<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// The name I2 is looked up in the same scope as I1</span>
                 <span class="co1">// that is, from the current scope, so it finds ::I2</span>
  AB x<span class="sy4">;</span>
  x.<span class="me1">AB</span><span class="sy4">::</span>~AB<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// The name AB after ~ is looked up in the same scope as AB before ::</span>
               <span class="co1">// that is, from the current scope, so it finds ::AB</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11">
<td>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1"><a href="qualified_lookup.html#Enumerators"><span class="tocnumber">1</span> <span class="toctext">Enumerators</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="qualified_lookup.html#Class_members"><span class="tocnumber">2</span> <span class="toctext">Class members</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="qualified_lookup.html#Namespace_members"><span class="tocnumber">3</span> <span class="toctext">Namespace members</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="qualified_lookup.html#See_also"><span class="tocnumber">4</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h4> <span class="mw-headline" id="Enumerators">Enumerators</span>
</h4>
<p>If the lookup of the left-hand side name comes up with an <a href="enum.html" title="cpp/language/enum">enumeration</a> (either scoped or unscoped), the lookup of the right-hand side must result in an enumerator that belongs that enumeration, otherwise the program is ill-formed.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
</tr>
</table>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Class members">edit</a>]</span> <span class="mw-headline" id="Class_members">Class members</span>
</h4>
<p>If the lookup of the left hand side name comes up with a class/struct or union name, the name on the right hand side of <code><b>::</b></code> is looked up in the scope of that class (and so may find a declaration of a member of that class or of its base), with the following exceptions
</p>
<ul>
<li> destructor name is looked up as described above (in the scope of the name to the left of ::)
</li>
<li> <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion</a> function name is <span class="t-mark">(TODO)</span>
</li>
<li> names used in template arguments are looked up in the current scope (not in the scope of the template name)
</li>
<li> names in <a href="namespace.html" title="cpp/language/namespace">using-declarations</a> also consider class/enum names that are hidden in current scope
</li>
</ul>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: micro-examples for the above </td>
</tr></table>
<p>If the right hand side of <code><b>::</b></code> names the same class as the left hand side, the name designates the <a href="constructor.html" title="cpp/language/constructor" class="mw-redirect">constructor</a> of that class. Such qualified name can only be used in a declaration of a constructor and in the <a href="using_declaration.html" title="cpp/language/using declaration">using-declaration</a> for an <a href="using_declaration.html#Inheriting_constructors" title="cpp/language/using declaration">inheriting constructor</a>. In those lookups where function names are ignored (that is, when looking up a name on the left of <code><b>::</b></code>, when looking up a name in <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier">elaborated type specifier</a>, or <a href="derived_class.html" title="cpp/language/derived class">base specifier</a>), the same syntax resolves to the injected-class-name:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> A<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span> B<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
A<span class="sy4">::</span><span class="me2">A</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// A::A names a constructor, used in a declaration</span>
B<span class="sy4">::</span><span class="me2">B</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// B::B names a constructor, used in a declaration</span>
B<span class="sy4">::</span><span class="me2">A</span> ba<span class="sy4">;</span>   <span class="co1">// B::A names the type A (looked up in the scope of B)</span>
A<span class="sy4">::</span><span class="me2">A</span> a<span class="sy4">;</span>    <span class="co1">// Error, A::A does not name a type</span>
 
<span class="kw1">struct</span> A<span class="sy4">::</span><span class="me2">A</span> a2<span class="sy4">;</span> <span class="co1">// OK: lookup in elaborated type specifier ignores functions</span>
                <span class="co1">// so A::A simply names the class A as seen from within the scope of A</span>
                <span class="co1">// (that is, the injected-class-name)</span></pre></div></div>
<p>Qualified name lookup can be used to access a class member that is hidden by a nested declaration or by a derived class. A call to a qualified member function is never virtual
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw1">virtual</span> <span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B <span class="br0">{</span> <span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    D x<span class="sy4">;</span>
    B<span class="sy3">&amp;</span> b <span class="sy1">=</span> x<span class="sy4">;</span>
    b.<span class="me1">foo</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls D::foo (virtual dispatch)</span>
    b.<span class="me1">B</span><span class="sy4">::</span><span class="me2">foo</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::foo (static dispatch)</span>
<span class="br0">}</span></pre></div></div>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Namespace members">edit</a>]</span> <span class="mw-headline" id="Namespace_members">Namespace members</span>
</h4>
<p>If the name on the left of <code><b>::</b></code> refers to a namespace or if there is nothing on the left of <code><b>::</b></code> (in which case it refers to the global namespace), the name that appears on the right hand side of <code><b>::</b></code> is looked up in the scope of that namespace, except that 
</p>
<ul><li> names used in template arguments are looked up in the current scope
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N <span class="br0">{</span>
   <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> foo <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
   <span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span>
N<span class="sy4">::</span><span class="me2">foo</span><span class="sy1">&lt;</span>X<span class="sy1">&gt;</span> x<span class="sy4">;</span> <span class="co1">// error: X is looked up as ::X, not as N::X</span></pre></div></div>
<p>Qualified lookup within the scope of a <a href="namespace.html" title="cpp/language/namespace">namespace</a> <code>N</code> first considers all declarations that are located in <code>N</code> and all declarations that are located in the <a href="namespace.html#Inline_namespaces" title="cpp/language/namespace">inline namespace members</a> of <code>N</code> (and, transitively, in their inline namespace members). If there are no declarations in that set then it considers declarations in all namespaces named by <a href="namespace.html#Using-directives" title="cpp/language/namespace">using-directives</a> found in <code>N</code> and in all transitive inline namespace members of <code>N</code>. The rules are applied recursively:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x<span class="sy4">;</span>
<span class="kw1">namespace</span> Y <span class="br0">{</span>
  <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">float</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> Z <span class="br0">{</span>
  <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> A <span class="br0">{</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> Y<span class="sy4">;</span>
  <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> B <span class="br0">{</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> Z<span class="sy4">;</span>
  <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> AB <span class="br0">{</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> A<span class="sy4">;</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> B<span class="sy4">;</span>
  <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
  AB<span class="sy4">::</span><span class="me2">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// AB is searched, AB::g found by lookup and is chosen AB::g(void)</span>
            <span class="co1">// (A and B are not searched)</span>
  AB<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// First, AB is searched, there is no f</span>
            <span class="co1">// Then, A, B are searched</span>
            <span class="co1">// A::f, B::f found by lookup (but Y is not searched so Y::f is not considered)</span>
            <span class="co1">// overload resolution picks A::f(int)</span>
  AB<span class="sy4">::</span><span class="me2">x</span><span class="sy2">++</span><span class="sy4">;</span>    <span class="co1">// First, AB is searched, there is no x</span>
              <span class="co1">// Then A, B are searched. There is no x</span>
              <span class="co1">// Then Y and Z are searched. There is still no x: this is an error</span>
  AB<span class="sy4">::</span><span class="me2">i</span><span class="sy2">++</span><span class="sy4">;</span>  <span class="co1">// AB is searched, there is no i</span>
            <span class="co1">// Then A, B are searched. A::i and B::i found by lookup: this is an error</span>
  AB<span class="sy4">::</span><span class="me2">h</span><span class="br0">(</span><span class="nu16">16.8</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// First, AB is searched: there is no h</span>
                <span class="co1">// Then A, B are searched. There is no h</span>
                <span class="co1">// Then Y and Z are searched.</span>
                <span class="co1">// lookup finds Y::h and Z::h. Overload resolution picks Z::h(double)</span>
<span class="br0">}</span></pre></div></div>
<p>It is allowed for the same declaration to be found more than once:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> A <span class="br0">{</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">namespace</span> B <span class="br0">{</span> <span class="kw1">using</span> <span class="kw1">namespace</span> A<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">namespace</span> D <span class="br0">{</span> <span class="kw1">using</span> A<span class="sy4">::</span><span class="me2">a</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">namespace</span> BD <span class="br0">{</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> B<span class="sy4">;</span>
  <span class="kw1">using</span> <span class="kw1">namespace</span> D<span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
  BD<span class="sy4">::</span><span class="me2">a</span><span class="sy2">++</span><span class="sy4">;</span> <span class="co1">// OK: finds the same A::a through B and through D</span>
<span class="br0">}</span></pre></div></div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: the rest of 3.4.3.2[namespace.qual], try to shorten their examples </td>
</tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<ul>
<li> <a href="unqualified_lookup.html" title="cpp/language/unqualified lookup">Unqualified name lookup</a> 
</li>
<li> <a href="scope.html" title="cpp/language/scope">Scope</a> 
</li>
<li> <a href="adl.html" title="cpp/language/adl">Argument-dependent lookup</a> 
</li>
<li> <a href="function_template.html" title="cpp/language/function template">Template argument deduction</a>
</li>
<li> <a href="overload_resolution.html" title="cpp/language/overload resolution">Overload resolution</a>
</li>
</ul>

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